Such cryostats are used, for example in infrared (IR) detectors and missile guidance systems which operate under cryogenic conditions. For many such applications, particularly for space-related applications, the vacuum of space may be utilized to reduce the vapor pressure over the cryogen below its normal boiling temperature, even to the point where the cryogen will freeze and cool the IR detector below the triple point of the cryogen, e.g., 63.degree. K. for N.sub.2 and 14.degree. K. for H.sub.2. For this purpose, it is necessary to collect the liquid cryogen in a matrix that will retain the cryogen while the pressure is reduced. Heat is transferred from the detector to the liquid or solid cryogen as it evaporates or sublimes. The matrix must be effective not only to collect and retain the liquid cryogen, but also to transfer heat from the detector to the cryogen in the matrix.
One matrix for collecting and retaining liquid and solid cryogen is described and claimed in my U.S. Pat. No. 5,012,650. This matrix comprises multiple layers of at least one highly adsorbent material and at least one relatively porous high thermal conductivity material. The highly adsorbent material, which may be described as a "wick" material, may comprise various materials with fine pores or fibers, including particularly glass fiber paper as well as polyester cotton. The porous high thermal conductivity material may conveniently comprise a wire mesh screen of 100 to 150 mesh for a small matrix, i.e. one centimeter thick, while a coarser wire screen is suitable for a larger matrix. Copper or aluminum, which have high thermal conductivity at cryogenic temperatures make good screen materials. The matrix may be formed as stacked alternate layers of wire mesh and glass fibers, or as a wound roll of wire mesh and glass fiber. Further details of the composition and construction of this storage matrix is described in this U.S. Pat. No. 5,012,650.
In one embodiment of a cryostat described in this U.S. Pat. No. 5,012,650, the storage matrix is contained in a reservoir directly connected to a Joule-Thompson (JT) heat exchanger. The gas/liquid mixture discharge from the restricted orifice of the JT heat exchanger is directed to inpinge against and flow over the top surface of the matrix material to cool the matrix and fill the reservoir with liquid cryogen. However, it has been found in practice that although liquid is generated at the orifice shortly after gas flow is initiated, the matrix is cooled and the reservoir is filled relatively slowly.
In a second embodiment also described in this U.S. Pat. No. 5,012,650, the storage matrix is contained in a separate reservoir which is connected to receive a separate source of cryogen gas. This reservoir is externally cooled by the heat exchanger. With this arrangement the matrix can be cooled and the reservoir filled more rapidly, but the cryostat assembly is considerably heavier, more complicated and more expensive.